A number of methods are known for producing porous silica-based particles, and representative examples thereof include (1) a method in which silica sol or the like is spray-dried with use of a spray dryer, and (2) a method in which silica-based particles are produced by an emulsion method.
For example, Patent Document 1 discloses a spray-drying method in which porous silica-based particles having an average particle diameter of 1 to 20 μm are produced by spray-drying a colloidal liquid containing primary particles having an average particle diameter of not more than 250 nm (i.e., fine silica-based particles) with use of a spray dryer.
Patent Document 2 discloses an emulsion method in which water glass (i.e., sodium silicate) is added into an organic solvent containing a surfactant such as an ester of sorbitan fatty acid to give an emulsion which is then neutralized to produce porous silica-based particles having an average particle diameter of 0.1 to 5,000 μm.
These patent documents, however, neither describe nor suggest the method for producing porous silica-based particles having a smooth surface as defined in the present invention.
Further, it is generally known that spherical porous silica-based particles and the like are blended as a feeling-improving agent into a cosmetic.
For example, Patent Document 3 discloses a method for producing a solid cosmetic such as a powdery foundation having high slippage and smooth feeling of use by blending porous silica-based particles into a cosmetic.
Patent Document 4 discloses that blending of spherical porous silica-based particles having a refractive index of 1.3 to 1.8 into a cosmetic improves spreadability of the cosmetic in use and uniformity of a cosmetic film.
These patent documents, however, neither describe nor suggest the cosmetic comprising porous silica-based particles having a smooth surface as defined in the present invention.
On the other hand, Non-patent Document 1 discloses that foreign matters such as small particles with a nanosize attached to the surface of commercially available silica-based particles are removed by an ultrasonic cleaning method to improve the surface smoothness of the particles. Non-patent Document 1 further describes that the silica-based particles thus obtained are suitable for applications of cosmetics such as foundations.
Even when the method using the ultrasonic cleaning is applied, however, foreign matters such as small particles with a nanosize will not be completely removed from the surface of the silica-based particles. Further, the porous silica-based particles per se are likely to be collapsed by the impact of ultrasonic waves, and it becomes difficult to obtain a large number of the particles with a high sphericity.
Patent Document 1: Japanese Patent Application Publication No. 270201/1986
Patent Document 2: Japanese Patent Application Publication No. 230609/1987
Patent Document 3: Japanese Patent Application Publication No. 60283/2005
Patent Document 4: Japanese Patent Application Publication No. 248971/2006
Non-patent Document 1: Toshiyuki KANI, Tokyo University of Agriculture and Technology 2007, Page 45-64